1. Field of the Invention
The present invention relates to an electron beam irradiating apparatus, and specifically to an electron beam irradiating apparatus which can irradiate an electron beam upon a plane scanned by an electron beam, by designating electron beam irradiation positions. Further, the present invention relates to an electric signal detecting apparatus incorporated in the electron beam irradiating apparatus, and more specifically to the detecting apparatus for generating a video signal in response to a photoelectric signal generated by the electron beam irradiating apparatus.
2. Description of the Prior Art
In the semiconductor manufacturing process for instance, the microstructure of a semiconductor sample is observed with the use of an electron beam irradiating apparatus such as an electron microscope, for instance.
In the prior art electron beam irradiating apparatus, a saw-tooth waveform signal referred to as a horizontal scanning signal and a vertical scanning signal is applied to a scanning coil to scan irradiation positions on a scanning plane in sequence continuously. Further, a horizontal blanking signal is applied to the scanning coil to return the electron beam to the start position for each scanning line. Additionally, whenever all the scanning operation ends once on one scanning plane, a vertical blanking signal is applied to the scanning coil, so that the same area can be irradiated with the electron beam repeatedly.
In the case where the semiconductor sample is observed in the semiconductor manufacturing process, the surface of the semiconductor sample is usually formed of an insulating substance such as photoresist. Therefore, when the sample as described above is continuously irradiated with the electron beam of the prior art electron microscope, there arises a problem in that the sample is charged up electrically because the conductive path of the irradiated electrons is small and narrow.
Once this charge-up phenomenon exists, since the irradiation direction of the electron beam applied upon the sample is distorted by the electric charge, an object to be observed by the electron microscope cannot be accurately photographed, or noise is superposed upon the object detection signals.
To reduce the electric charge accumulated on the sample, various countermeasures have been so far proposed, for instance such that the voltage for accelerating the electron beam is lowered (low voltage method); the electron density is reduced (low doze method); the scanning speed is increased (high speed scanning method), etc. In the above-mentioned prior art methods, however, it is still extremely difficult to hold all over the scanned sample surface in an equipotential level.
One of the major reasons is that in the prior art electron beam irradiating apparatus, the line scanning is repeated, except a special case as in an automatic focusing operation. In this repeated line scanning operation, as far as the conductive sample is being scanned, there exists no specific problem. However, when the insulating sample is being scanned repeatedly, the electric charge up increases in proportion to the number of repeated scanning operation. When the scanning width is as narrow as several pixels in particular, the charged electrons can be well discharged, so that the electric charge is not accumulated, in the case of where the scanning width extends to about 50 pixels, with the result that it becomes difficult to observe the sample of insulating material.
Further, in the prior art electric signal detecting apparatus incorporated in the electron beam irradiating apparatus, when the electric signal such as photoelectric signals are integrated by the sample hold circuit in response to the pixel clock signal of the video signal within the period of the clock signal, since the number of the sample hold circuit is only one, there exists a problem in that it is impossible to integrate the photoelectric signal over a sufficient time interval within the period of the clock signal, because the same sample hold circuit must respond to the succeeding clock signal.